Conventionally, as a color space to indicate RGB data, there has been a method that uses sRGB color space which is a standard color space for monitors. For example, upon JPEG (Joint Photographic Experts Group) data which is commonly used is opened on a personal computer, the sYCC data recorded in the JPEG data is immediately converted to sRGB data. If data outside of the sRGB gamut exists in the sYCC data (i.e., negative value or value above 256 with an 8-bit value), natural clip to the sRGB color space occurs, and color phase shift can occur.
Natural clip indicates a phenomenon wherein a color outside the gamut of the image data supplied externally is forcibly expressed with a color within the gamut of the device. For example, in the case that only the R-component of a color shown with RGB is greater than the maximum value of the R-component of the gamut, the R-component of the color thereof is expressed with the maximum value of the R-component of the gamut (natural clip). At this time, the RGB balance of the original color is lost due to this natural clip, and the hue changes. This type of hue change is called color phase shift. That is to say, the color is expressed with a color different than the original color, so this sort of color phase shift occurrence is not favorable.
In order to prevent this, there is a method to perform gamut conversion which compresses the gamut of the image data into the sRGB gamut on the recording device side beforehand. With this gamut conversion, the original sYCC data has already been compressed within the sRGB gamut, so even if the JPEG data is opened and subjected to sRGB conversion, data outside the gamut is not generated. That is to say, the occurrence of color phase shift as described above can be suppressed.
However, while sRGB represents the gamut of a standard personal computer monitor, there are portions that are decisively narrow when compared with the gamut of a printing device or wide gamut liquid crystal television receiver (wide gamut liquid crystal TV) or the like.
FIG. 1 is a schematic diagram showing a comparison state of gamut ranges. As shown in the example in FIG. 1, a gamut 1 of a general inkjet printer is narrower than an sRGB gamut 2 in many hues, but many of the blue/green regions are wider. Also, a gamut 3 of a wide gamut liquid crystal TV is generally wider that the sRGB gamut 2 in all hues.
However, once the gamut of moving picture content is converted to the narrow sRGB gamut, information before compression that had been originally recorded is lost. This information is then unable to be expressed, regardless of the gamut of the output device. Expression is impossible, even with a wide gamut liquid crystal TV having a wide gamut. That is to say, with gamut conversion to a narrow gamut, image quality of the moving picture content may unnecessarily be deteriorated.
Therefore, various methods to restore the data that has once been compressed, and compress again into the gamut of the output device, have been proposed (e.g., PTL 1 and PTL 2).
PTL 1 discloses a method where barcode information showing profile data is printed onto paper along with the image, and by scanning the barcode information thereof in the event of printing with a separate printing device, gamut compression is performed again for the separate printing device.
Also, PTL 2 discloses a method where information before compression of the compressed RGB data (maximum/minimum values, compression table, and the like) is read in, image data is restored, and gamut compression is performed again for the final output device.